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Of course, this is a joke problem. It would be impossible for 60,000 men to build a similar wall because most of them could not get near to the wall because of the crowd. It is a warning that written problems do not always make sense in real life.
Laura from West Flegg Middle School in Norfolk made it 5.76 seconds (this assumes the 4 men building the original wall worked a 24 hour day)!
This is as long as it takes to read this solution. Can that be right?